If I removed wp-foro and reinstalled there is a chance that it will be ok? I understand that I will lose all forum posts?

But it sure comes from something outside wpForo stock plugin and Wordpress stock script.

https://snipboard.io/VqIBNQ.jpg

However, when I remove the html code from here:

 "; s: 44:" wp_new_user_notification_email_admin_subject "; s: 32:"  New User Registration "; s: 44:" wp_new_user_notification_email_admin_message "; s: 95:" New user registration on your site :

Username: 

Email:  "; s: 38:" wp_new_user_notification_email_subject "; s: 42:"  Your username and password info "; s: 38:" wp_new_user_notification_email_message "; s: 207:" Username: 

<span class = \ "tlid-translation translation \" lang = \ "pl \"> <span class = \ "\" title = \ "\"> To set a password, visit the following address </span> </span> :

 "; s: 28:" reset_password_email_message ";

And I am trying to register again,  emails do not reach the addressee at all :((

Ok

I did register to your site and got a problematic email as you say above.

Due to some "translation" plugin or method you use.

<span class=\"tlid-translation translation\" lang=\"pl\">
<span class=\"\" title=\"\">Aby ustawić hasło, odwiedź następujący adres</span></span>:

http://niepodam.pl/ - login "test888" - you can see html:
https://snipboard.io/B0dIUt.jpg

The same situation is in Gmail:
https://snipboard.io/j6YXbs.jpg
and everywhere else ...

Now I can see that the headers are correct in Gmail:

MIME-Version: 1.0
Content-Type: text / plain; charset = UTF-8
Username: Kamila123

But I still have html tags in emails. Someone in the previous thread said that it is not the plugin fault, but the fact that I am sending the wrong content-type, but it is probably the fault of the wpforo script ?

<span class = \ "tlid-translation translation \" lang = \ "pl \"> <span class = \ "\" title = \ "\"> To set a password, visit the following address </span> </span> :
<Https://girlinneed.com/forum/?wpforo=resetpassword&rp_key=1Phzo7swOcTDg852mwA9&rp_login=Kamila123>

How do I get rid of those damn html tags?
It's killing me ...

MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8

and works right now.

What type is showing in your message header? And what client is that?

https://wpforo.com/community/how-to-and-troubleshooting-2/activation-link-is-broken-registration-key-is-invalid/

Apparently the problem was in wp_mail_content_type (must be plain text not html), so I changed it according to the instructions, and added in the functions.php file:

function wps_set_content_type(){
return 'text/plain';
}
add_filter( 'wp_mail_content_type','wps_set_content_type' );